The performance of an integrated circuit metal oxide semiconductor (MOS) transistor depends on a number of device parameters such as gate dielectric thickness, transistor gate length, and the mobility of the electrons and/or holes in the MOS transistor channel region. The mobility of the electrons and/or holes (herein after referred to as carriers) is a measurement of how quickly the carriers traverse the transistor channel region. In general, the mobility of the carriers in the transistor channel region is related to the velocity of the carriers and the channel electric field by μ=Vcarriers/Echannel, where μ is the carrier mobility, Vcarriers is velocity of the carriers in the channel, and Echannel is the electric field in the MOS transistor channel. In general, the carrier mobility is affected by a number of factors including the scattering of the carriers as they traverse the transistor channel region from the transistor source region to the transistor drain region.
An important measure of MOS transistor performance is the magnitude of the transistor drain current (IDS) obtained for a given gate-source voltage (VGS) and a given drain-source voltage (VDS). In addition to being dependent on VGS and VDS, IDS is proportional to the carrier mobility μ. It is therefore important that the carrier mobility μ be maximized for improving transistor performance. Recently, it has been found that the application of stress in the transistor channel region is an important factor in increasing the value of the carrier mobility μ. A number of methods have been utilized to apply stress to the transistor channel region including the formation of a high stress film over the transistor structure. It has been found that the applied stress is a function of the thickness of the film, with the applied stress increasing with film thickness. The high density of integrated circuits limits the thickness of the films that can be used. There is therefore a need for a method to increase the stress produced in the transistor channel by high stress films without increasing the film thickness. The instant invention addresses this need.